The invention relates to a cathode ray display tube of the type having a rectangular display screen, an electron gun system to generate at least one electron beam, and a deflection unit is connected on the display tube in such manner that their longitudinal axes coincide. The deflection unit comprises a set of line deflection coils which, upon energization deflect the electron beam in a first direction and a set of field deflection coils which upon energization deflect the electron beam in a direction transverse to the first direction. The sets of deflection coils upon energization, generate a dynamic magnetic multipole field comprising at least a dipole component and a sixpole component.
In monochrome cathode ray display tubes the electron gun system is adapted to generate one electron beam incident to the display screen, whereas in colour display tubes the electron gun system is designed to generate three electron beams which converge on the display screen. The description hereinafter will for the sake of simplicity relate to the deflection of one electron beam.
The deflection unit for deflecting the electron beam is used to deflect the electron beam in one or in the other direction from its normal undeflected straight path, so that the beam impinges upon selected points on the display screen to provide visual presentations. By varying the magnetic deflection fields in a suitable manner, the electron beam can be moved upwards or downwards and to the left or to the right over the display screen. By simultaneously modulating the intensity of the beam a visual presentation of information or a picture can be formed on the display screen. The deflecting unit attached to the neck portion of the cathode ray tube comprises two sets of deflection coils enabling deflection of the electron beam in two directions which are transverse to each other. Each set comprises two coils which are arranged on oppositely located sides of the tube neck, the sets being shifted relative to each other through 90.degree. about the tube neck. Upon energization the two sets of deflection coils produce orthogonal deflection fields. The fields are essentially perpendicular to the path of the undeflected electron beam. A cylindrical core of a magnetizable material which closely engages the sets of deflection coils when the two sets of deflection coils are of the saddle type, is used to concentrate the deflection fields and to increase the flux density in the deflection area.
In order to satisfy certain requirements regarding picture quality, the (dynamic) magnetic deflection fields should often be modulated strongly. For example, the stringent convergence requirements in three-in-line colour television systems necessitate, in addition to a strong positive magnetic sixpole component on the gun side of the field deflection field, a strong negative magnetic sixpole component in the centre of the field deflection field. Monochrome display systems of high resolution require, in addition to a positive magnetic sixpole component on the screen side of both the line and the field deflection field, a negative magnetic sixpole component in the centre for good spot quality. In systems having a large deflection angle it is particularly difficult to realize the required modulations by only the wire distribution of the sets of deflection coils, if possible at all, the deflection unit often becomes too expensive.